1. Field of the Invention
The present invention relates to a continuously variable transmission (CVT), and more specifically to a straddle-type vehicle equipped with an electronically controlled CVT.
2. Description of Related Art
In straddle-type vehicles, such as scooter motorcycles, a V-belt type continuously variable transmission (CVT) is widely used. A V-belt type CVT includes a primary shaft to which output of a power source, such as an engine, is input, a secondary shaft that extracts output to be transmitted to a driving wheel, and a paired primary sheave and a secondary sheave that are respectively disposed on the primary shaft and the secondary shaft. The groove width of each of the sheaves is designed to be variable, and a V-belt is wound around the sheaves. The V-belt type CVT has a groove width adjustment mechanism that is used to vary the groove width of each sheave. As a result, the winding diameter of the V-belt around each sheave is adjusted to adjust the speed change ratio between the sheaves in a continuously variable manner.
The primary and secondary sheaves are each formed by a fixed flange and a movable flange that form a V groove therebetween. Each movable flange moves in an axial direction of the primary or secondary shaft. The groove width adjustment mechanism moves the moveable flange to adjust the speed change ratio in a continuously variable manner.
A known V-belt type CVT of this type uses an electric motor to move a movable flange of a primary sheave, thereby adjusting the groove width. The movement driving force of the electric motor moves the moveable flange in either a direction that narrows the width of the primary sheave (a Top side) or a direction that widens the width of the groove of the primary sheave (a Low side), thereby allowing adjustment of the groove width (for example, refer to Japanese Patent No. 3043061).
A scooter motorcycle provided with a mechanism for electronically controlling a V-belt type CVT automatically changes the speed change ratio without requiring the rider to perform any operations, based on a program (map) that has been input in advance for the vehicle speed and the engine speed. Accordingly, the rider's driving operation is simpler. Attempts have been made to apply this automatic CVT to various types of vehicles.
When a vehicle equipped with such a CVT runs down a downward slope, the vehicle may coast down the downward slope by inertia with the engine stopped. In this case, if a mechanism that changes the speed change ratio in response to the vehicle speed is used, when the engine is started after a certain speed is reached, the clutch may be engaged immediately. At this time, a problem may occur, namely, an unpleasant sensation may be felt because of an inconsistency between the rider's operation and the actual acceleration movement of the vehicle.
As shown in, for example, FIG. 9, a CVT 1 includes a primary sheave 3 connected to a primary shaft 3a that is rotated by an engine 2, a secondary sheave 4 connected to a secondary shaft 4a that outputs power to a rear (driving) wheel 7 via a centrifugal clutch 6 and a V-belt 5 wound around primary sheave 3 and secondary sheave 4. A groove width adjustment mechanism adjusts the groove width of primary sheave 3, thereby continuously adjusting the speed change ratio.
CVT 1 may include a mechanism for changing the speed change ratio in response to the vehicle speed when the vehicle runs down a downward slope with engine 2 stopped. In this case, when engine 2 is started in a state where a certain speed has been reached, an event may occur where shifting is rapidly performed based on a program (map) that has been input in advance for the vehicle speed and engine speed. When such an event occurs, there is a possibility that the rotation speed of secondary sheave 4 located downstream of engine 2 will also increase, thus instantaneously engaging centrifugal clutch 6. As a result, the feeling in this case is completely different from that in the case of clutch engagement when starting normally. As a result, an unpleasant sensation may be felt because of an inconsistency between the rider's operation and the actual vehicle movement. The inventors of the present invention have found that this unpleasant sensation caused by the inconsistency between the rider's operation and the vehicle movement may be felt depending on the engagement state of centrifugal clutch 6, and have devised a mechanism that reduces the unpleasant sensation being felt when the centrifugal clutch is engaged, thus achieving the present invention.